This invention relates to a superconductor having a high critical temperature and a high critical electrical current and to a method of producing same.
Recent development of a melt processing technique has enabled the production of excellent superconductors. One such melt processing technique is a melt-powder-melt-growth (MPMG) process by which a superconductor containing a Y.sub.2 BaCuO.sub.5 (Y211) phase dispersed in a YBa.sub.2 Cu.sub.3 O.sub.y (Y123) phase and having a high critical electrical current can be obtained ("Melt Processed High Temperature Superconductor", M. Murakami, World Scientific, 1993). Such a superconductor can create a large electromagnetic force upon interaction with a magnetic field and may be utilizable for bearings, flywheels, transporting devices, etc.
Another known melt processing technique is a process in which a large crystal, such as of Y123, having controlled crystal orientation is grown using a seed crystal such as of La123, Nd123 or Sm123. A large crystal with controlled crystal orientation is desirable for application to magnetic shield and permanent magnet.
Known RE123 (RE: rare earth element) superconductors have a serious problem that part of the Ba site is substituted by RE ion which has a large ionic radius so that the critical temperature is lowered (H. Uwe et al, Physica C, vol. 153-155, p. 930-931 (1988)).
With regard to a sintering method, there is a proposal in which a RE123 superconductor is produced in a nitrogen atmosphere and the resulting product is subsequently oxidized so as to prevent the lowering of the critical temperature (T. Wada et al, J. Am. Ceram. Soc., 72, 2000-2003 (1989)). The superconductor thus prepared is, however, still unsatisfactory and cannot be put into practice, because the superconducting transition occurs through a wide temperature range and because the volume fraction of the superconducting phase is small. This is perhaps attributed to the fact that the sintering method, which utilizes a solid state reaction, is susceptible to an influence of the solid solution region and solid state diffusion for homogenization of Nd on Ba sites is very sluggish so that substitution of Ba ion with RE ion significantly occurs.